A, Cell viability assays evaluating the response of PLK1 and NOTCH inhibitors. Percent of live A375, SK-MEL-2, and SK-MEL-28 cells treated by different doses of PLK1 inhibitor BI 6727 (BI) and/or NOTCH inhibitor MK-0752 (MK) are shown as mean ±SEM of at least three biological replicates in duplicates. The combination index (CI) for to each treatment is listed below the dose. B, Representative images of colony formation assays. A375, SK-MEL-2 and SK-MEL-28 melanoma cells treated with PLK1 inhibitor BI 6727 and/or NOTCH inhibitor MK-0752 followed by colony staining using crystal violet of at least three biological replicates in duplicate. *P <0.05, **P <0.01, ***P <0.001, and ****P<0.0001. DMSO: dimethyl sulfoxide; BI: BI 6727; MK: MK-0752; CI: Combination Index.